News Main Menu

Pseudorabies in PA Rabies

Nancy Marie Brown

December 1, 2004

Pseudorabies in PA Rabies

There are some 7,600 pig farms in Pennsylvania, containing from a few to 12,000 pigs. Lancaster and Lebanon counties have the densest pig populations and the biggest problem with the swine virus called pseudorabies; there, between 1992 and '94, 91 herds were under quarantine for pseudorabies infection.

"I've been to a lot of the Pork Producers' meetings," says Heather Norman, a graduate student studying the epidemiology of the disease to aid the Pennsylvania Department of Agriculture's current eradication program. "The producers seem to want to cooperate," she says, "but many of them are distrustful of government programs."

Home of scrapple and Lebannon bologna: A map of pig population in Lancaster & Lebanon Counties.

With reason: In the past, the state's approach to controlling the disease was "herd depopulation." Farmers had to start over with clean pigs, usually at their own cost. "Reinfection," says Norman, "was all too common, because no one was sure how the virus spread between herds." The answer is still unclear. Some researchers believe it can travel long distances through the air. It may survive temporarily in soil or water. Although most wild animals are killed by it outright, there's a suspicion rodents can harbor the virus and spread it to pigs. Flies may also be a vector.

"Some people think it can't be eradicated," says Norman, "that it will never be."

Like chicken pox (which provokes a similar fatalistic response from the parents of pre-schoolers), pseudorabies is a herpes virus—not, as its name would imply, a form of rabies. It's more a nuisance than a health threat. In suckling pigs it can be fatal; in weanlings and older pigs, it tends only to slow down their fattening and leave them susceptible to other infections. It cannot be passed to humans. "A lot of outbreaks now are subclinical," Norman explains. "The producer may not know the pigs are infected, especially if there are no young animals on the farm."

Yet it represents an economic threat: "The pig doesn't fatten well. It has to stay on the farm longer." Time equals feed equals money—which is why the state is eager to wipe pseudorabies out: In Pennsylvania, pigs are a $126 million industry.

To discover why some farms get pseudorabies and others don't, Norman and her adviser, assistant professor of veterinary science William Sischo, used a computer Geographic Information System to plot each of 123 quarantined pig herds as the center of a two-mile-radius circle; 162 herds that had never been quarantined were plotted as controls. Using data on 511 herds in Pennsylvania, the computer system then calculated the numbers of pigs, herd types, and infected herds within each circle.

"In general," Norman concludes, "the more crowded your area is, the higher the risk of your herd getting pseudorabies. But just because you're in a high-density area doesn't mean your pigs are definitely going to become infected."

Neither is having a quarantined herd in the neighborhood, in itself, a risk factor. In fact, the statistics hinted that having a quarantined herd inside the two-mile zone might even confer a slight protective effect. "While this seems counterintuitive," Norman and her collaborators write, "it is possible that once producers know they are infected they implement control measures, thereby decreasing amounts of circulating virus."

These results mean, Norman adds, that "herd biosecurity is important, that producers can do something to protect their herds."

The true risk factors may have more to do with the number of trucks, visitors, and other pigs coming onto the farm than with the health of the neighbors' herds. "Do you make visitors change their shoes?" Norman asks farmers. The questionnaire she and Sischo sent out also suggests other infection routes: Are all new pigs vaccinated? Are breeding boars borrowed or lent? Are the trucks which come to take pigs to market already partly loaded with pigs from other farms? Do the truck drivers help load the pigs? Can dogs, cats, or wildlife get in with the pigs?

"This study looked at characteristics of the neighbors' herds in relation to the herds in the center," Norman explains. "Next we want to look at the center herds to try to identify the risk factors."

Heather Norman is a master's student in the department of veterinary science, College of Agricultural Sciences, University Park, PA 16802; 814-865-7696. Her adviser, William Sischo, Ph.D., is assistant professor of veterinary science; 863-8526. Collaborating with them were USDA veterinarians Amy Nesselrodt and Paul Pitcher; Suzette Thompson of the Pennsylvania Department of Agriculture; and Rick Day of Penn State's Department of Agronomy. The project was funded by the Pennsylvania Pork Producers Council and the Pennsylvania Department of Agriculture.